Tuñacao whipped Kumarnthong with his effective jab and body shots from the start to finish to get the nod of all the three judges. The official scores were 120-109, 119-109, 118-111.
The IBF ranked No.4 and WBC No.8 Tuñacao, who is now managed by Japanese businessman Yasuo "Machang" Matsuoka, improved his win-loss-draw record to 19-1-1 spiked with 13KOs since turning pro in August 1998.
It was Tuñacao's third straight victory this year in Japan - all via stoppage. He earlier pummeled Takafumi Himeno into ninth round submission, before stopping Ryoichi Hidaka in the seventh round last July 18 at the Central Gym in Osaka, Japan.
Tuñacao hasn't fought in the Philippines since beating Cagayan de Oro City's Bernard Uclos by decision on September 19 last year at the Mandaue City Sports and Cultural Complex.